(1) Field of the Invention
The present invention relates to the detection of the presence of sheet material and particularly to ascertaining whether a strip of material, which is to be or has been bonded to an underlying sheet of material, is being fed in proper alignment with the underlying sheet. More specifically, this invention is directed to apparatus for providing a warning signal in the absence of thin sheet material and especially to apparatus for sensing the presence of a strip of thin transparent foil which, for example, is to be bonded to a sheet of foil. Accordingly, the general object of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well-suited for use in packaging and particularly in the wrapping of products, such as cigarettes, which are hermetically sealed to insure freshness. As is well-known, cigarette packages are wrapped in a transparent, for example, polypropylene, foil. This wrapping is typically provided with a tear strip or band to facilitate opening of the packages. The tear-open band is usually comprised of a transparent material and is welded or glued, in the wrapping machine, to the transparent foil. Since a faulty package would result from the absence or breakage of the tear-open band, means must be provided to stop the wrapping machine if the tear-open band breaks or the supply thereof to the wrapping machine is interrupted for any reason. The failure of the tear-open band may be easily detected during its delivery to the point where it is brought into abutting relationship with the wrapping foil. For example, the band may be passed over a compensating roller which is connected to a suitable sensor, the sensor detecting either the failure of the roller to rotate or the absence of force on the roller.
Continuing to discuss the cigarette packaging application, faulty wrapping will also occur when the tear-open band departs from the desired alignment, i.e., the tear-open band will be skewed relative to a desired feed direction. Any such misalignment may result in the tear-open band failing to pass the gluing or welding device by which it is bonded to the wrapping foil. Such misalignment is difficult to detect. For example, a transparent tear-open band cannot be detected optically. The cigarette industry has, accordingly, previously resorted to use of visual inspection, by attendance personnel, to insure that a tear-open band is being properly delivered into abutting relationship with a wrapping foil. It has long been desired to automate this monitoring procedure, particularly since the attendance personnel typically have other tasks to perform and it is thus possible for a substantial number of faulty packages to be formed before a tear-open band feed problem has been noted and corrected.
Apparatus for controlling the position of the edge of a fabric or paper sheet, wherein plural rolls mounted independently of one another are employed, is known in the art. An example of such prior apparatus may be seen from German patent specification No. 736,136. In this type of prior art device the sheet of material, the position of which is to be controlled, moves under the rollers, the rollers individually controlling the state of electrical contacts. Should the sheet of material not be present under at least one roller, the state of the associated contacts will change and a fault signal will be generated. In response to this fault signal, the edge of the sheet of material may be shifted sideways to an appropriate extent while the material feed speed is reduced. This type of prior art control apparatus would not, however, be applicable to determining the presence and direction of delivery of a tear-open band which is being brought into abutting contact with an underlying sheet.